In this report we identify an accessory cell that interacts with primed and memory T cells at sites where they collaborate with B cells. These cells are distinguished from conventional dendritic cells by their lack of response to Flt3 ligand and their inability to process antigen. Unlike dendritic cells, the CD4(+)CD3(-) cells have little CD80 or CD86 expression but do express high levels of the TNF ligands, OX40 ligand and CD30 ligand. We show that Th2-primed cells express the receptors for these TNF ligands and preferentially survive when cocultured with these cells. Furthermore, we show that the preferential survival of OX40(+) T cells and support of memory T cell help for B cells are linked to their association with CD4(+)CD3(-) cells in vivo.

Mice rendered deficient in CD28 signaling by the soluble competitor, cytotoxic T lymphocyte–associated molecule 4–immunoglobulin G1 fusion protein (CTLA4-Ig), fail to upregulate OX40 expression in vivo or form germinal centers after immunization. This is associated with impaired interleukin 4 production and a lack of CXC chemokine receptor (CXCR)5 on CD4 T cells, a chemokine receptor linked with migration into B follicles. Germinal center formation is restored in CTLA4-Ig transgenic mice by coinjection of an agonistic monoclonal antibody to CD28, but this is substantially inhibited if OX40 interactions are interrupted by simultaneous injection of an OX40-Ig fusion protein. These data suggest that CD28-dependent OX40 ligation of CD4 T cells at the time of priming is linked with upregulation of CXCR5 expression, and migration of T cells into B cell areas to support germinal center formation.

SummaryImmunoglobulin (Ig) class switch recombination is associated with the production and splicing of germline IgCH messenger RNA transcripts. Levels of'y1 transcripts in mouse spleen sections were assessed by semiquantitative analysis of reverse transcriptase polymerase chain reaction (PCk) products during primary and secondary antibody responses to chicken gamma globulin (CGG). This was correlated with the appearance of CGG-specific B cells and their growth and differentiation to plasma cells. After primary immunization with CGG, ~/1 switch transcripts appeared after 4 d, peaked at a median of six times starting levels between 10 and 18 d after immunization, and returned to background levels before secondary immunization at 5 wk. By contrast, after secondary challenge with CGG, a sevenfold increase in transcripts occurs during the first d. The level again doubles by day 3, when it is six times that which is seen at the peak of the primary response. After day 4, there was a gradual decline over the next 2-3 wk. Within 12 h of secondary immunization, antigen-specific memory B cells appeared in the outer T zone and by 24 h entered S phase, presumably as a result of cognate interaction with primed T cells. Over the next few hours, they migrated to the edge of the red pulp, where they grew exponentially until the fourth day, when they synchronously differentiated to become plasma cells. The same pattern was seen for the migration, growth, and differentiation of virgin hapten-specific B cells when CGG-primed mice were challenged with hapten protein. The continued production of transcripts after day 3 indicates that switching also occurs in germinal centers, but in a relatively small proportion of their B cells. The impressive early production of switch transcripts during T cell-dependent antibody responses occurs in cells that are about to undergo massive clonal expansion. It is argued that Ig class switching at this time, which is associated with cognate T cell-B cell interaction in the T zone, has a major impact on the class and subclasses of Ig produced during the response.